The d100 System uses a single core mechanism for all action resolutions. It originated with a set of house rules for Stormbringer but diverged widely into a generic, universal system capable of supporting any genre.

Characters have eight Statistics (attributes) including Strength, Constitution, Size, Dexterity, Intelligence, Power, Charisma, and Sanity. Scores can be generated randomly or purchased from a point pool, and will run from 1-100. Statistics are then modified for character Race (Elf, Dwarf, Halfing, Half Orc, Half Elf, Orc, Troll, or Human). Characters have seven Derived Statistics. Characters have Racial Abilities and select a Career Path which provides access to Skills. Combat utilizes a variety of random tables to determine effects, including fumbles and criticals. Combat supports a wide variety of special maneuvers. The game includes a developed magic system.

The game usually is distributed with two PDF character sheets--a fantasy/generic sheet, and a modern sheet.